The neuromuscular strategies used for the control of trunk equilibrium in the seated posture have received little attention in the motor control literature. Yet maintaining and controlling the seated posture is of basic interest in terms of human motor control, is of interest in the field of man-machine interactions in ergonomics and has considerable physical, psychological, functional, social and economic ramifications for a number of groups of non-ambulatory patients. The objective of this research is to first describe and quantify how trunk equilibrium is controlled in the healthy, seated individual using a combined experimental and theoretical approach to study a range of static, quasi-static and dynamic tasks. To do this kinematic, myoelectric and external reaction force data will be measured in standardized test procedures. Biomechanical models will be used to predict, on the basis of the underlying mechanics, what trunk, hip and lower extremity muscle activity patterns should be required while performing the tasks in the manner observed. Correlations comparing predicted and measured spatio-temporal muscle recruitment patterns will be used to validate the models, and to further refine them. These models will then be used to predict how one class of non- ambulatory, trunk and leg muscle-impaired patients, paraplegics, will perform in the same tasks. Again muscle activity predictions will be correlated with the kinematic and myoelectic data gathered in these patients. Theoretical and experiemental studies of the effect of external postural supports will also be conducted. This research should give a theoretical framework with which to improve the diagnosis and treatment of a much wider group of non-ambulatory patients, once we understand better how trunk equilibrium is maintained in the healthy individual and one class of patient with a clearly defined lesion.